I can think of many uses. It will be really usefull on multiplayer servers. Admins will be able to write server rules, console commands or help notes into few books and than place them in spawning area. Admins will no longer have to cover entire walls with signs.

Could be something similar to enchanting.
Adventure map creators could give them a specific power so when players open the book, they are given a new ability. (Increased XP drops, improved strength etc)